In general, coal is partial oxidized in a partial oxidation furnace and resultant gas is introduced into a gas turbine to produce the electric power. It has also been considered to use heavy oil instead of coal. In the partial oxidation furnace, oxidation using a less amount of oxygen than that stoichiometrically required for complete combustion of a raw material, i.e. so-called incomplete combustion, is carried out, and the produced gas (mixed gas) contains hydrogen, carbon monoxide, steam and a little carbon dioxide, and further contains impurities such as hydrogen sulfide (H2S), COS, HCN and so on.
As one of methods for removing the impurities contained in the foregoing mixed gas, a wet absorption method using methyldiethanolamine (MDEA) as an absorption solvent is known. However, although this method is effective for selectively removing H2S, the rate of removal of another impurity, i.e. COS, is low, and further, HCN reacts with MDEA to form a stable compound so that MDEA is degraded. Therefore, it is necessary to remove COS and HCN in a pretreatment before implementing the wet absorption method.
In order to perform the foregoing pretreatment, it has been considered to use a catalyst for converting COS into H2S that is liable to be selectively absorbed by MDEA and for converting HCN into ammonia that is harmless to MDEA, through conversion reactions represented by reaction formulae (1) to (3) given below.COS+H2O→CO2+H2S  (1)HCN+H2O→NH3+CO  (2)HCN+3H2→NH3+CH4  (3)
As the foregoing catalyst, an alumina catalyst carrying Group VI metal and barium reported in JP-A-2000-51694 or an alkalized chromium oxide-alumina catalyst carrying alkali metal and chromium oxide reported in JP-A-2000-86211 is known, for example.
When converting COS and HCN in the mixed gas using the foregoing catalyst, there arise the following problems. As carriers of alumina catalysts, γ-alumina (Al2O3) having a relatively high activity is used, for example. However, γ-alumina phase can change into boehmite (AlOOH) in the presence of steam at a high partial pressure with, for example, a temperature of the mixed gas being 180° C. and a steam partial pressure being 0.4 MPa. This would lower the activity of the catalyst due to the reduction of the BET specific surface area effective for the activity of the catalyst.
If the activity of the catalyst is reduced as noted above, the mixed gas with a high concentration of COS and HCN is sent to an absorber that implements removal of H2S, so that, as described above, the absorption solvent is degraded to lower the rate of removal of H2S. Further, in case of producing the electric power by means of a gas turbine that uses the mixed gas as fuel, there is concern about emitting sulfur oxides (SOx) and nitrogen oxides (NOx) being acid gas into the air.
Even in case of the alkalized chromium oxide-alumina catalyst, lowering of the activity of the catalyst due to phase transition of alumina into boehmite is recognized when the catalyst is used over a long term, and further, it is confirmed that potassium is scattered from the catalyst with a lapse of time. Therefore, it is necessary to use an expensive material such as an alloy 825 or 625 for preventing alkali corrosion, as a material of an apparatus such as a cooler that is provided downstream of a reaction apparatus using the catalyst, so that there is concern about increase in cost of initial investment and maintenance.
The present invention has been made under these circumstances and has the object to providing a means for decomposing/removing COS and HCN contained in mixed gas produced by partial oxidation, using a catalyst that is excellent in steam-resistant property and highly active for converting COS and HCN.